1. Field of the Invention
The invention relates to a photoelectric converting apparatus and, more particularly, to a one-dimensional or two-dimensional photoelectric converting apparatus for executing a reading operation of, for example, a facsimile, a digital copying apparatus, an X-ray image pickup apparatus, or the like. More specifically, the invention relates to a removal of a random noise which is caused by a resetting operation of a photosensor.
2. Related Background Art
Hitherto, a CCD has mainly been used as an image pickup device of an image reading system such as facsimile, digital copying apparatus, or the like, a video camera, a digital camera, or the like. In recent years, however, a development of what is called an amplifying type photoelectric converting apparatus having an amplifying function of an MOS transistor or a bipolar transistor on a pixel unit basis is also vigorously being executed.
In an amplifying type photoelectric converting apparatus, in order to realize a high sensitivity, a removal of a noise becomes important. With respect to the noise removal, however, several methods have conventionally been proposed.
FIGS. 6A and 6B are circuit diagrams of a one-dimensional photoelectric converting apparatus having a transistor as a photosensor for each pixel and a timing chart for such a circuit (refer to the magazine of The Institute of Television Engineers of Japan, Vol. 47, No. 9, pp. 1180, 1993).
The operation of such a photoelectric converting apparatus will now be described hereinbelow. When a start pulse (SP) is inputted, accumulating capacitors CTS and CTN are reset and a photosignal including a sensor noise is subsequently transferred to the capacitor CTS.
After that, a resetting operation of a photosensor is executed and an output in a dark state including the sensor noise is transferred to the capacitor CTN. The sensor is again reset and an accumulating operation is started. At the same time, a shift register of the first chip starts to scan and data of CTS and CTN is respectively capacitance divisionally outputted to CHS and CHN as common output lines of the first chip. The outputted signals are respectively inputted to a differential amplifier through voltage followers, so that a signal without any sensor noise is derived as an output of an IC.
xe2x80x9cSensor reset noisexe2x80x9d used here denotes an FPN (Fixed Pattern Noise) which is caused due to a variation in hFE of a transistor serving as a photosensor of each pixel or a variation in capacitance Cbc, between a base and a collector. Namely, an emitter potential after an emitter was reset by xcfx86ERS varies due to the variations in hFE and Cbc, every pixel. Therefore, it appears as an offset in the ordinary reading operation. However, since the offset can be removed by such a method, the FPN can be reduced.
As another prior art, s solid-state image pickup apparatus as shown in FIGS. 7 and 8 has been proposed in Japanese Patent Application Laid-Open No. 1-154678.
In the diagram, signal charges which were caused by an incident light and accumulated in each photodiode 101 are outputted by the following procedure. At the start of a horizontal blanking period of an output of the apparatus, when the photodiodes 101 of one line in the horizontal direction to be read out next are selected, a reset line 106 corresponding to such a line is turned on or off. After a resetting operation was performed by a reset switch 103, when a drain line 107 is subsequently turned on, each pixel amplifier 104 of such a line operates as a driver transistor of a source follower. An output of each source follower in this instance is an amplifier output when there is no signal charge. By turning on/off a gate line 116, the output voltage is stored into an accumulating capacitor 111 through a gate switch 109.
Subsequently, when a vertical gate line 105 corresponding to such a line in the horizontal direction is turned on or off and signal charges are supplied to a gate of each pixel amplifier 104, the output of each source follower has a value corresponding to an amount of signal charges. By turning on or off a gate line 117, the output voltage is stored into an accumulating capacitor 112 through a gate switch 110.
The operation in the horizontal blanking period is executed as mentioned above. In a horizontal scanning output period of time, a horizontal register 122 sequentially on/off scans horizontal gate switches 113 and 114 corresponding to each pixel, so that the source-follower output charges accumulated in the accumulating capacitors 111 and 112 are sequentially outputted from a horizontal signal line 120.
The output charges accumulated in the capacitors 111 and 112 are obtained by time-sequencing outputs in both of the case of resetting and the case of inputting the signal charges with regard to one pixel amplifier 104. Further, by obtaining a difference between both of those outputs, a noise which is caused by a variation in input offsets of a plurality of source followers and a 1/f noise of the source followers can be easily suppressed.
However, the prior art mentioned above has a problem to be solved such that the reset noise which is caused upon resetting of a photoelectric conversion unit cannot be removed.
Each time the photosensor is reset, an electric potential which was reset fluctuates and a reset noise appears as a random noise.
For example, in a photodiode having a pn junction, when a light production carrier Qp is accumulated into a capacitor CPD of a photodiode unit and converted into a voltage, a photosignal voltage Vp by the light production carrier is
VP=QP/CPDxe2x80x83xe2x80x83(1)
On the other hand, since a reset noise VN is
VN=(kT/CPD)xe2x80x83xe2x80x83(2)
where, k: Boltzmann""s constant
T: Temperature (xc2x0 K.)
an S/N ratio is
VP/VN=QPDxc2x7(1/(kTCPD))xe2x80x83xe2x80x83(3)
Therefore, from the equation (3), in order to raise the S/N ratio, although it is desirable to reduce the accumulated capacitance CPD of the photosensor as small as possible, there is practically a limitation. On the other hand, since there is a tendency such that the signal charges Qp decrease in accordance with the realization of a high fineness and a high speed of the photoelectric converting apparatus, the removal of the reset noise becomes an important point when realizing a high S/N ratio of the photoelectric converting apparatus.
However, in the prior art 1, as shown in the timing chart of FIGS. 6A and 6B, the sensor is reset twice and the photosignal and noise signal to be read out are based on the different sensor resetting operations. Namely, after completion of the second sensor resetting operation, the accumulating operation and the photosignal reading operation are executed. A sensor noise N which is generated by the first sensor resetting operation is subtracted from a read-out signal (S+Nxe2x80x2) including a sensor noise Nxe2x80x2 generated by the second sensor resetting operation, thereby removing the sensor noise. Therefore, a random noise that is 2 times as large as the sensor reset noise is generated.
In the prior art 2 as well, since the resetting operation by the reset switch 103 is executed in a closed state of the gate switch 109, the resetting operation of the photodiode 101 and the resetting operation by the reset switch 103 are different. Therefore, a random noise which is caused due to the resetting operation cannot be perfectly removed in a manner similar to the prior art 1.
In the prior art 2, although an example in which the gate switch 109 is not provided is also disclosed, there is a disclosure such that there is a problem such that the random noise due to the sensor resetting operation is also generated even in such a case.
Namely, in the conventional techniques, although the FPN can be improved, the random noise due to the resetting operation of the photosensor is not improved yet.
It is an object of the invention to reduce a random noise due to a resetting operation of a sensor unit in a one-dimensional or two-dimensional photoelectric converting apparatus in which a photosignal accumulation and a signal output are simultaneously executed.
As means for solving the foregoing problems, it is an object of the invention to provide a photoelectric converting apparatus for outputting a signal during an accumulation of a photosignal, comprising:
photoelectric conversion means;
reset means for resetting the photoelectric conversion means;
noise signal holding means for holding a noise signal when a resetting operation of the photoelectric conversion means is executed; and
noise signal removing means for removing a noise from the signal accumulated in the photoelectric conversion means after completion of the same resetting operation by using the noise signal held.
According to the invention, there is also provided a photoelectric converting apparatus comprising:
a plurality of photoelectric conversion means arranged in a one-dimensional or two-dimensional form;
reset means for resetting the plurality of photoelectric conversion means;
noise signal read-out means for reading out noise signal charges just after the resetting operation;
photosignal read-out means for reading out photosignal charges after the photosignal was accumulated; and
scanning means for sequentially scanning the noise signal of the noise signal read-out means and the photosignal of the photosignal read-out means,
in which the signals are read out from the noise signal read-out means and the photosignal read-out means by the scanning means and, at the same time, the photosignal is accumulated in the photoelectric conversion means,
wherein the photoelectric converting apparatus further comprises
a noise signal holding unit for holding the noise signal just after the resetting operation at least means a timing before the photosignal after completion of the same resetting operation is read out by the photosignal read-out means, and
for outputting a difference between the noise signal just after the resetting operation and the photosignal after completion of the same resetting operation.
According to the invention, by providing the noise holding means, after the noise generated upon resetting of the sensor was held, the accumulating operation and reading operation of the photosignal are executed as they are and the difference between the noise and the photosignal in the same sensor resetting operation can be obtained, so that a random noise which is caused upon sensor resetting can be perfectly removed.
The above and other objects and features of the present invention will become apparent from the following detailed description and the appended claims with reference to the accompanying drawings.